1. Field of the Invention
The present invention relates to a method and an apparatus for manufacturing thermoset webs of thermoplastic polymer material which is comminuted into particles, such as flakes, granulated material or the like or is ground into powder.
2. Description of the Related Art
For manufacturing webs of thermoplastic polymer materials comminuted into particles, flakes, granulated material or the like, it is known in the art to pour the particles or a mixture of different particles continuously as a surface-covering initial layer onto a conveyer belt and to press and bond this initial layer with a successively or simultaneously carried out application of heat and pressure into a web, wherein the web is subsequently immediately cooled.
In addition, for manufacturing webs of plastic material with patterns, it is known in the art to roll a mixture of flakes of several plastic mixtures having different colors into a web or to roll the mixture of flakes into a single-color web of a thermoplastic mixture, and subsequently to combine one or more of these prepatterned webs and a web having the basic color of the prepatterned web or webs of a compatible thermoplastic mixture with each other by the application of pressure and heat, and to calender the combined webs in the hot state while reducing the thickness thereof.
A manufacturing method of this type is described, for example, in DE-AS 19 28 405. However, this method has the significant disadvantage that only webs having a relatively small thickness of a few millimeters can be produced since the rolling of webs having greater thicknesses above 5 mm causes substantial difficulties because of the danger of bubbles or air inclusions. In addition, the step of calendering resulting in the reduction in the material thickness inevitably also produces an aligned structure of the pattern of the webs in the calendering direction. In the case stronger calendering takes place there is the additional danger that the structure of the pattern is more or less blurred by the formation of mixed colors. This effect of mixed colors and premixing becomes even more pronounced when using particles with different melting viscosities.
Therefore, for manufacturing the webs with non-aligned patterns, techniques have been developed which avoid rolling or calendering as the last step of the manufacture of the web or plates; these techniques are described, for example, in DE-OS 14 79 090 and DE-AS 18 79 822.
In the method mentioned last, the thermoplastic material flakes are uniformly layered into a continuous stack and are heated in the stack, wherein the stack is then cut into a crude block and is compressed in a box die. The desired plates are then cut from the pressed block. This method makes it possible to obtain patterned plates of compact thermoplastic material with non-oriented patterns even with the formation of only small portions of mixed colors. However, this method has the disadvantages that it operates discontinuously which is expensive, and that various dies and long cooling times of the pressed blocks are required so that an economic manufacture is not possible.
A method of continuously manufacturing webs with non-aligned patterns using thermoplastic material particles is disclosed in EP 0 046 526. This method also makes it possible to manufacture webs having greater compact thicknesses of 1.5 to 10 mm, preferably 4 to 8 mm, while avoiding air inclusion and without requiring rolling and calendering procedures. This method specifically provides that the raw material layer composed of the thermoplastic material particles travels through a heating zone and is preheated in the heating zone and the layer is then continuously pressed as it travels through a treatment area, preferably with the use of a double-swing band press, in such a way that the layer is compacted and bonded in a first phase with the application of a pressure of preferably 10-60 bars and heating to a temperature in the range of 160-200.degree. C., and, in a second and immediately subsequent phase, the layer is cooled while maintaining the pressure and is simultaneously pressed to the predetermined thickness. The continuously carried out surface pressure according to this method for compacting and bonding the thermoplastic material particles into a web avoids the disadvantages of the linear pressure application in the case of rolling or calendering of webs and also the conventional discontinuous pressing over sections which was used in the past. However, this method has the disadvantage that, on the one hand, the apparatus for carrying out the method is complicated and, on the other hand, that the method produces a compacted web with surfaces compacted excessively so as to reduce the elasticity thereof, wherein this is caused by the surface pressure applied with high pressure over the entire treatment area.